Osmosis (AQA A Level Biology)

The Process of Osmosis

• All cells are surrounded by a cell membrane which is partially permeable

• Water can move in and out of cells by osmosis

• Osmosis is the diffusion of water molecules from a dilute solution (high concentration of water) to a more concentrated solution (low concentration of water) across a partially permeable membrane

• In doing this, water is moving down its concentration gradient

• The cell membrane is partially permeable which means it allows small molecules (like water) through but not larger molecules (like solute molecules)

Osmosis and the partially permeable membrane

• Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane

• Water potential describes the tendency of water to move out of a solution. This term is used to avoid confusion between water concentration and concentration of a solution

  • A dilute solution has a high water potential (the right-hand side of the diagram below) and a concentrated solution has a low water potential (the left-hand side of the diagram below)

How osmosis works

• The water potential of pure water (without any solutes) at atmospheric pressure is 0kPa, therefore any solution that has solutes will have a water potential lower than 0kPa (it will be a negative value)